Electron multiplier spurious noise baffle



May 9, 1961 D. c. DAMo'rH ETAL 2,983,845

ELECTRON MULTIPLIER SPURIOUS NOISE BAFFLE Filed May 7. 1959 2 Sheets-Sheet 1 -3200 VOLTS POWER SUPPLY -ISOOVOLTS 500 VOLTS INVENTORS DONALD c. DAMoTH GEORGE w. GooDRlcH WILLIAM c.w|LEY ATTORNEY May 9', 1961 D. c. DAMoTH ETAL ELECTRON MULTIPLIER SPURIOUS NOISE BAFFLE 2 Sheets-She 2 Filed May 7, 1959 V//l/l'l/l/l, 36

FIG-4 FIG-5 INVENTOR. DONALD C. DA MOTH GEORGE w. GooDRlcH BY WILLIAM c. w|| EY ATTORNEY ELECTRON MULTIPLIER SPURIOUS NOISE BAFFLE -Filed May 7, 1959, Ser. No. 811,751

'1'1 Claims. (Cl. 315-85) Y This invention pertains to an electron multiplier spurious noise baille to reduce noise in an electron multiplier.

lt is an essential object of this invention to provide a spurious noise baflle for reducing noise in an electron multiplier which is believed due to electron, ion, photon or other type feedback.

It is another object to provide in an electron multiplier having a secondary emissive surface and a related eld surface spaced therefrom to dene an electron path, a spurious noise baffle which surrounds such surfaces to block particles believed to have escaped from between the surfaces and to be traveling in a feedback path to a point of entry or re-entry between the surfaces and impingement on the emissive surface, and to block particle products believed on an entry or re-entry path.

It is a further object of invention to provide in an electron multiplier having a pair of secondary electron emissive surfaces and electrical and magnetic ields aligned relative to said surfaces to urge charged particles in a cycloidal path between said surfaces, a spurious noise bathe comprising a 'section surrounding said surfaces in a plane substantially perpendicular to the surfaces, which section is closely spaced to the surfaces and is believed to intercept and prevent particle travel in a reverse or irregular manner outside of said surfaces.

These and other objects will become more vapparent when preferred embodiments :of the invention are considered in connection with the drawings in which:

. Figure 1 is a perspective View, partly in block form, schematically illustrating a magnetic electron multiplier constituting one embodiment of this invention;

Figure 2 is Va plan viewillustrating the electrical iield produced in the multiplier shown in Figure l and show. ing the bale in section;

`Figure 3 is a plan view illustrating electron travel in the multiplier of Figure l; 4 Figure 4 is =a section taken along 4 4 of Figure 3 showing thenoise baffle; and l Figure 5 is -a plan view of a second embodiment having a plurality of such baliies with thecentral baifle being sectioned.

A problem experienced in the art of designing and manufacturing electron multipliers and` like instruments having a vacuum or gaseous path for electron ilow, has been that of spurious or noise signals. It is believed that one of the sources of such spurious signals isthat of particles and particle products, such as electrons which escape from the defined path between two secondary electron emissive surfaces, gas molecules ionized by electrons, photons generated by `high energy particles, or other mass or energy particles which travel rearwardly or irregularly to re-enter or enter such defined path causing a spurious or false output signal. :This invention overcomes the problem by constructing a balfle, or bafes, intermediate the ends of the secondary electron emissive surfaces 'to reduce the spurious or unwanted noise signals. Itis believed that the baffle structure surrounding the secondary emissive surfaces in a plane perpendicular United States Patent ICC 2 to the surfaces, blocks such particle and product travel outside the path deiined by the secondary surfaces,y so that particles escaping from between the surfaces and products of these particles cannot re-enter or enter the surfaces at an earlier or later point in the cycle to generate noise signals.

` Referring now to the drawings, a first embodiment of this invention will be described. A vacuum tube 9 encloses a source for emitting electrons, such as a cathode 10, which is positioned to introduce electrons to a magnetic electron multiplier generally indicated at 12. The multiplier 12 includes a pair of substantially parallel plates 14 and 16 which are spaced from each other a distance such as 1%: inch. The plates 14 and 1-6 are made of an insulating material such as glass or Bakelite.

The plate 14 is provided with a conductive coating or strip 18 on its inner surface and a similar conductive strip 20 is provided on the inner surface of the plate 16. The conductive strips 18 and 20 are made of a secondary electron emissive material having a relatively high resistance such as atin oxide or carbon compound. Since the secondary emissive properties of the strip 18 in this embodiment are not utilized, if manufacturing economies may be achieved thereby, the strip 18 may be made from a material which 'has a relatively high. resistance but whichdoes not have outstanding secondary emissive properties.

Terminals 22 and 24 are provided on the opposite ends of the plate 14 in contact with the conductive strip 18. Similarly, terminals 26 and 28 are provided on the opposite ends of the plate 16 in contact with the conductive strip 20. I'he terminals 22, 24, 26 and 28 are made of a conductive material having a very low resistance, such as silver.

A baille 29, shown in section in Figure 2, and having a central opening through which pass the plates 14, 16,v extends from the plates in a plane perpendicular' to the' plates. Satisfactory results have been obtained with the baffle 29 being made of an electrically conductive rna terial (stainless steel) or an insulative material (polytetrafluoroethylene resin). I-t is believed any material suiciently impenetrable to electron, ion, or photon travel is usable. The boundary of the central opening in they baille is dimensioned as close to the outerA surfaces kof glass plates 14, 16 as possible without interfering with the normal multiplying action therebetween. l.

An anode plate 30 is disposed in substantially per-Q pendicular relationship to the plates 14 and 16'to receive any electrons passed between the plates. The anode 30 is connected through a resistance 32 to a power supply 34 which applies a direct voltage such as +1500 volts to the anode.

Direct voltages, such as +1500 volts and d500/volts?. are applied respectively to the terminals 22 and 24 from" stantially parallel tothe face of the plates. vForrexample, a magnetic eld of 300 gausses may be provided by the' pole piecesv t The application of the direct voltages to .thel terminals 22 and 424 produces a ow of currentfthroughgthefcon-I ductiye strip 18. .The amount of wcllrlent flow isY relatively'lsmallbecause of the high resistance of the strip 18. For exampleha 1current* of one `milliampere maybe produced'in the strip 18. This current ilow results in a uniform voltage drop across the conductive strip 18 b e- 3 'tween the terminals 22 and 24. A similar current flow and uniform voltage drop results across the conductive strip 20 between the terminals 26 and 28.

Although the magnitude of the voltage drop across the strips 18 and 20 is substantially the same, the drop in each strip occurs between different potential levels. This causes the equipotential lines between the strips 18 and 20 to be slanting. For example, equipotential line 1500 volts is represented by the slanting line 40 (Figure 2) which is drawn between the terminal 24 and an intermediate position in the strip 20. Since the electric field in a region is disposed in a direction perpendicular to the equipotential lines in the region, the electric field shown in Figure 2 is produced in the region between the strips 18 and 20 in a direction perpendicular to the equipotential line 40. This field has a component substantially perpendicular to the plates 14 and 16 in a direction for causing any electrons in the region to move towards the plate 14. The field also has a component substantially parallel to the plates 14 and 16 in a `direction to cause any electrons inthe region lto 4acquire energy in their travel to the anode 22. In this Way the electrons acquire suflicient energy to cause secondary emission at a ratio greater than 1:1 when they impinge upon a surface of the plates.

Electrons emitted by the cathode 10 are subjected to the combined action of the magnetic field and electric field in the region between the plates 14 and 16. This causes the yelectrons to travel in a cycloidal path 50 and to strike the surface of the strip 20 which emits a proportionately increased number of electrons. The electrons emitted by the strip Z travel in a cycloidal path 52 and impinge upon another part of the strip 20 to again emit a proportionately increased number of electrons for cycloidal travel to another part of the strip 20. In this way, successively emitted electrons travel across the surface of the strip 20 in successive cycloidal paths as shown in Figure 2 to multiply the number of electrons initially emitted by the cathode 10. Finally, the electrons emitted by the strip 20 at its extreme left impinge upon the anode 30 for detection.

As mentioned, it is believed that in the operation of the disclosed multiplier, electrons generated by the multiplier escape from between plates 1'4, 16 and travel in an irregular or reverse manner outside the confines of the plates 14, 16. These charged particles are believed to re-enter the area between plates 14, 16 striking an emis- Sive surface 18, 20 to generate false signals on anode 30. Also, some ions, photons, or other energy or mass particles formed or present in the multiplier envelope are believed to enter between the plates 14, 16 to generate false or spurious signals. Placement of baille 29 approximately midway between the ends of plates 14 and 16 has effectively reduced the noise and it is believed the reason for this is that the stray particles and products are blocked before they can re-enter the multiplier path and strike one or the other of the emissive strips 18, 20 to generate a number of spurious or noise electrons which are, of course, amplified in the normal manner as they pass between plates 14 and 16 to cause a noticeable divergence from the true amplification.

A second embodiment is shown in Figure and is similar to the embodiment shown in Figures 1 to 4 except that three batles 54, 56 and 58 are placed, respectively, at the input end, at the center, and the output end of the plates 14 and 16. The baille may take forms other than -a section in a perpendicular plane and may be of other suitable materials and locations.

Although this invention has been disclosed and illustrated with reference to particular applications, the principles involved are susceptible of numerous other applications which will be apparent to persons skilled in the art. The invention is, therefore, to be limited only as indicated by the scope of the appended claims.

Having thus `described our invention, we claim:

1. An electron multiplier comprising means for producing primary electrons, secondary electron emissive means, means for establishing an electric field having a component perpendicular to said emissive means, means for establishing a magnetic field having a component substantially perpendicular to the electric field so that a charged particle in said elds will follow a cycloidal path, envelope means for lenclosing the secondary emissive means, a baille structure'being in said envelope means and closely placed relative to said emissive means and having a surface component perpendicular to said emissive means to reduce noise and spurious signals.

2. The multiplier of claim l wherein said baflle structure comprises a section substantially encircling said secondary electron emissive means.

3. The multiplier of claim 2 wherein said section is in a plane substantially normal to the secondary electron emissive means and is centrally placed longitudinally of said emissive means.

4. The multiplier of claim 2 wherein said baille section 4has an opening through which said secondary emissive means passes, the inner boundary of the baille section opening being closely spaced relative the emissive means the outer boundary of said baille fitting closely against said envelope and electric eld means in order to reduce particle and product travel outside of said emissive and field means.

5. The multiplier of claim l which is further characterized by said baille structure comprising a plurality of sections spaced longitudinally of said emissive means, each of said sections having a component extending in a plane perpendicular to the longitudinal direction of said emissive means.

6. The multiplier of claim 5 which is further characterized by a section being placed adjacent each end of said emissive means and centrally of said emissive means.

7. The multiplier of claim 6 where at least one of said baille sections is close fitting -against said envelope and is closely spaced' to said emissive means.

8. The multiplier of claim 2 wherein said baffle section has an opening through which said secondary emissive means passes, the inner boundary of the baille section opening being closely spaced relative the emissive means, the outer boundary of said baille fitting closely against said envelope in order to reduce particle and product travel outside of said emissive means.

9. A charged particle multiplier comprising secondary emissive means for yemitting a plurality of secondary charged particles upon impact of a primary charged particle, envelope means for enclosing the secondary emissive means, electric field means for establishing an electric field for accelerating said charged particles relative to said emissive means, a baille structure being in said envelope means and closely placed relative to said emissive means, and having a surface component perpendicular to said emissive means to reduce noise and spurious signals.

l0. The multiplier of claim 9 wherein said bafiie structure comprises a section substantially encircling said secondary electron emissive means.

y1l. 'Ihe multiplier of claim 10 wherein said baille section has an opening through which said secondary emissive means passes, the inner boundary of the baffle section opening being closely spaced relative the emissive means, the outer boundary of said baille fitting closely against said envelope and electric field means in order to reduce particle and product travel outside of said emissive and field means.

References Cited in the file of this patent UNITED STATES PATENTS 2,216,267 Flechsig Oct. l, 1940 2,231,691 Snyder Feb. ll, 1941 2,291,767 Shore Aug. 4, 1942 

